Probe instruments for sensing temperatures are used in liquid level detectors in which the level of the liquid in a tank is sensed by the probe. The probe is inserted into a tank and it is internally heated. When liquid covers the probe, heat transfer from the probe to the liquid cools the covered portion of the probe. Sensors in the probe determine which part of the probe is hot, and which is cold, and thus determine the level of the liquid. These probes and associated circuitry are basically detecting the heat transfer out of the probe which is a function of the level of the liquid in the tank. However, this type of probe could be used in other applications in which other conditions of the probe environment would affect the heat transfer. For example, such probe would be useful in equipment that detects a fluid flow profile across a fluid stream.
Numerous measuring instruments with probes have been developed for level detectors and related applications using a variety of structural configurations. One popular configuration uses a wire having a series of thermocouple junctions spirally wound about a rod extending vertically in a tank. One purpose of the spiral winding is to produce closely spaced thermocouple junctions in a vertical direction, and another purpose is to physically separate every other thermocouple junction. This separation is accomplished by arranging the spiral and the spacing between the junctions so that every junction is on an opposite side of the rod from its two adjacent junctions. Thus, two rows of thermocouple junctions are formed along opposite sides of the rod, and every other thermocouple junction may be heated by a linear heater cable disposed adjacent to one row of the junctions.
This type of spiral wound structure is inherently difficult to produce because the spiral requires precise spacing and positioning of the thermocouple junctions. The thermocouple junctions must be precisely spaced in the wire prior to being spirally wound about the rod and the spiral winding must be done with care so that the junctions will line up on the rod. If the spiral is too tight or too loose, the junctions will drift out of line. If the wire containing the thermocouple junctions is drawn after the junctions are formed therein, the drawing process must be precise to obtain the desired spacing along the wire between the junctions. The necessity of spirally winding the wire also requires that the thermocouple junction wire and its insulation or other protective coverings be flexible enough to bend around the rod. The spiral also introduces a vertical stagger between heated and non-heated junctions that may require either structural or electrical correction depending upon the application. When the cost of producing such spirally wound probe is held down, the trade-off is a loss of accuracy caused by the inherent difficulties in manufacturing this type of probe.